In a combined effort to develop an efficacious, moderately priced vaccination schedule against AIDS, three attenuated viruses approved and widely used as live vaccines, yellow fever virus (YFV), varicella zooster virus (VZV) and measles virus (MV) are applied as vaccination vectors to express SIV antigens. Later, when a combination of expressed SIV antigens has been found providing optimal protection of macaques against challenge with SIV, the vectors expressing analogous HIV antigens will be constructed and tested again in on-human primates for immunity induction and protection against hybrid (SHIV) and HIV viruses. The subgroups located at the University of Zurich, Switzerland and the Institute Pasteur, Paris, France, will be concerned with vector constructions based on attenuated MV, which usually mediates life-long protection against measles by single-application. MV has been shown to stably maintain and express added genetic material encompassing altogether more than 5000 nucleotides, inserted in three different genome positions, over many generations; this is unusual for RNA vaccines, inserted in three different genome positions, over many generations; this is unusual for RNA viruses, which usually tend to rapidly eliminate genetic material not essential for virus propagation. First, SIV proteins will be expressed as before from one of three transcription unit cloning cassettes added at different genome locations mediating differential expression levels. Second, an alternative expression strategy will be explored, linking SIV ORFs in such a way to the resident (essential) MV reading frames that cessation of full length SIV ORF expression, e.g. by adventitious formation of a premature stop codon, results in suicide of the vector. Third, replacement of the MV envelope proteins by functionally similar glycoproteins of other viruses, including SIV env, will be attempted to circumvent neutralization of the vector by antibodies present in individuals immune against measles. Propagation efficiency, expression levels of SIV antigens and genetic stability of the constructed vectors will be tested in cell culture and genetically modified mice infectable with MV.